make nfs_atomic_open() call d_drop() on all ->open_context() errors.

In "NFSv4: Move dentry instantiation into the NFSv4-specific atomic open code"
unconditional d_drop() after the ->open_context() had been removed.  It had
been correct for success cases (there ->open_context() itself had been doing
dcache manipulations), but not for error ones.  Only one of those (ENOENT)
got a compensatory d_drop() added in that commit, but in fact it should've
been done for all errors.  As it is, the case of O_CREAT non-exclusive open
on a hashed negative dentry racing with e.g. symlink creation from another
client ended up with ->open_context() getting an error and proceeding to
call nfs_lookup().  On a hashed dentry, which would've instantly triggered
BUG_ON() in d_materialise_unique() (or, these days, its equivalent in
d_splice_alias()).

Cc: [email protected] # v3.10+
Tested-by: Oleg Drokin <[email protected]>
Signed-off-by: Al Viro <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>

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