lockd: preserve multiple NLM_SHARE grants from the same owner

When an NFSv3/NLM client issues multiple NLM_SHARE calls from a single
host for the same (file, owner) tuple, the current implementation
overwrites the recorded access and deny modes with the latest pair.
A subsequent NLM_UNSHARE then drops the entire entry, even if other
grants were implicitly subsumed by the most recent SHARE.  This is
particularly visible to Windows-style clients that map each open of
a file to a distinct NLM_SHARE, all carrying the same NLM owner
handle.  For example:

    1. SHARE(access=RW, deny=W)   -> entry [RW, deny W]
    2. SHARE(access=R,  deny=N)   -> entry [R, deny N]   (RW/W overwritten)
    3. UNSHARE(access=R, deny=N)  -> entry freed
    4. UNSHARE(access=RW, deny=W) -> nothing to release

NLM has no duplicate reply cache, so both SHARE and UNSHARE handlers
must be idempotent under UDP retransmit.

Track each (access, deny) pair with a single bit in a u16 bitmap.
fsh_access and fsh_mode are each in {0..3}, so there are 16 possible
pairs; index = (access << 2) | deny.  SHARE sets the bit, UNSHARE
clears it, both via idempotent bit operations.  s_access and s_mode
are recomputed as the union of the (access, deny) values whose bit
is set, and the entry is freed once s_access_deny_bmap reaches zero.

NLM_UNSHARE gains the access and deny modes as arguments so the
correct bit can be cleared.  The two callers in svcproc.c and
svc4proc.c are updated to forward the decoded values.

Signed-off-by: Oscar Ou <oscarou@synology.com>
Link: https://patch.msgid.link/20260703063856.2423734-1-oscarou@synology.com
Signed-off-by: Chuck Lever <cel@kernel.org>

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